The invention relates to a distance-measuring method for determining the spatial dimension of at least one target and a corresponding rangefinder.
A multiplicity of measuring apparatuses have been known since antiquity for recording properties of defined points in a measuring environment, in particular of data having a spatial reference. The location of a measuring device together with any reference points present, and direction, distance and angle to targets and measuring points, are recorded as standard spatial data.
A generally known example of such measuring apparatuses is the theodolite. In the two-dimensional representation of the optical image, it is possible to specify points to which a measurement, i.e. the determination of distance and/or angle, is effected. On the basis of the image, targets can be identified and tracked by image processing methods, so that automated surveying on this basis is in principle possible.
However, this image has no depth information at all, so that the image processing methods are dependent on corresponding prior information, image recording conditions, such as, for example, the preorientation of a target plate, or image properties, such as, for example, brightness and contrast. The possibilities of target identification and target tracking are limited by the purely visual detection. In particular, it is not possible to resolve optical ambiguities as occur, for example, in the case of curved surfaces. Thus, a disc and a sphere may appear as an identical image in both cases in a frontal photograph under unfavourable light conditions.
In many applications, however, additional information about the spatial extension of objects is also relevant, not only individual selected points but the extensive form of the object being of interest. This applies, for example, to safety technology where, together with object detection, object recognition is often also required in order to initiate suitable safety measures. Further applications are in the industrial sector, for example in quality control in manufacture and in vehicle safety technology.
In the prior art there are various approaches to the determination of the spatial dimensions or the form of objects. For distance measurement, a generally optical signal is directed from the measuring system in the direction of the object to be surveyed and a part of the light reflected by the object is detected by the optical sensors of the measuring system and processed. From the phase shift or the signal transit time from the optical source of the measuring system to the object and back to the sensors, conclusions can be drawn about the distance of the respective measured point.
For this purpose, the distance to a plurality of points of the object is measured sequentially or simultaneously with the aid of an individual rangefinder moved in a scanning manner with respect to the beam path or a multiplicity of rangefinders which are arranged in a line or in an array or in another geometry adapted to the application, so that the object geometry can be reconstructed from the measured data obtained.
Systems using a scanning beam have disadvantages which result from the mechanical design. Either the total device must be moved over the visual area to be recorded or the beam guidance must be made variable with otherwise invariable apparatus. In addition to the cost for such mechanically and/or optically demanding or complex solutions, they mostly have only a low scanning speed and moreover possess a comparatively high energy consumption. Owing to the scanning movement and mechanical loads, for example due to vibrations, the correlation of the distance measurements with the image points of the visual image cannot be ensured or can be ensured only at additional cost.
Parallel data recording, i.e. the simultaneous measurement to a plurality of measured points or many measured points, is based on simultaneously carrying out a plurality of measuring processes, which also requires the duplication of components, for example by the use of a plurality of rangefinders. Advantageous solutions make use of integration of the sensors in an ASIC, permitting lower assembly cost of the system components and optimised manufacture.
The parallel approach can also be supported by a scanning mechanism, with the result that the lateral point density can be increased, or which also makes it possible to scan an object in all spatial dimensions using a linear arrangement of the sensors. This mechanism can be carried out, for example, on an electro-optical beam deflection, a mechanical beam deflection by means of mirrors or a manual beam deflection in combination with an inertial sensor for direction determination.
By parallelizing the measurement, the light source has to meet higher requirements compared with a single point measurement if, for example, comparable requirements regarding the measuring range and the uncertainty of measurement are assumed. In addition, a high degree of parallelizing can be expediently achieved technically only with a measurement concept whose realisation involves little complexity.
For example, the approach disclosed in DE 44 40 613 uses sinusoidally modulated light emitted by LEDs for distance measurement. The sensors arranged in an array scan the light reflected by the object with at least four scanning times. The phase difference between emitted optical signal and optical signal reflected by the object is determined from these by simple arithmetic. The phase is calculated after reading out the scanning values outside the sensor array. With a knowledge of the wavelength of the light modulation, the distance to the measured point can be derived from the measured phase.
By limitation to a few scanning points and dispensing with calculations in the sensors, these have little complexity. However, rangefinders with sinusoidally modulated light have poor sensitivity, which limits the use of the system to nearby objects or a low degree of parallelizing.
Similarly, in DE 197 04 496, the relative phase of a back-scattered signal is determined relative to the phase of a sinusoidally modulated light signal by detection with so-called photonic mixer devices. Here, the light sensor produces demodulation of the light received. By using this demodulation in various phase relationships between the emitted light signal and the demodulation signal, the phase between the emitted light signal and the received light signal, and hence the distance to the object, can be determined. Once again, poor sensitivity is achieved by the use of sinusoidally modulated light.
Brian Aull, “3D Imaging with Geiger-mode Avalanche Photodiodes”, Optics & Photonics News, May 2005, pp. 42ff, discloses a concept which uses pulsed light emitted by a laser for distance measurement. The back-scattered photons are detected as light-sensitive elements in single-photon detectors. The time of arrival of these photons is the measure for the distance to the respective object point. This time of arrival is determined by stopping a fast digital counter or a plurality of phase-shifted counters on detection of the photon. These counters have a relatively low degree of complexity. In addition, the measurement is very sensitive owing to the use of pulsed light. However, digital counters achieve poor resolution, which is determined by the clock interval. The method is therefore unsuitable for precision measurements.
C. Niclass, A. Rochas, P. A. Besse, E. Charbon, “A CMOS 3D Camera with Millimetric Depth Resolution”, IEEE Custom Integrated Circuits Conference, pp. 705-708, October 2004, discloses a similar method. However, a higher time resolution is achieved by using time-to-digital converters (TDC) which interpolate the photon arrival time within the clock interval. However, the high complexity of the TDC does not permit the use of a dedicated TDC for each sensor element. An individual TDC therefore detects the time of arrival of the first photon of a larger quantity of photons detected by sensors. All subsequently detected photons cannot be detected by the TDC and are therefore lost. This constitutes considerable limitation for the sensitivity of the system.
U.S. Pat. No. 6,115,112 describes a distance measuring instrument which determines the time-of-flight of pulsed laser light in order to derive the distance to an object point. Measurement is divided into a coarse measurement and into a fine measurement. During the coarse measurement, strong optical pulses are emitted so that the signal reflected back by the object and received is clearly distinguished from the existing noise. In the coarse measurement, the distance to the object point is roughly determined. In the subsequent fine measurement, the measuring signal—or a measuring product derived therefrom—in the vicinity of the time determined during the coarse measurement is scanned and further processed.
The method is not suitable for parallelized measurement using a plurality of sensors, since the requirement that the back-scattered signal differs clearly from the noise during the coarse measurement is achievable in practice only with great pulse energies and hence very complicated optical sources.